The goal of this proposal is to determine the function of vaults. Vaults are the largest cytoplasmic ribonucleoprotein particle known. Their considerable abundance and striking evolutionary conservation among eukaryotes argue for an important general cellular function. The existence of this particle was first reported 10 years ago and this grant has been the primary source of support for vault studies since it was first funded in 1987. In addition to identifying structure, composition, tissue and cellular distribution of this particle, past studies have also produced valuable reagents that will be essential for elucidating vault function. The hypothesis that vaults function in nucleocytoplasmic transport has received recent attention from investigators working on the mechanism of resistance of cancer cells to chemotherapy. Multidrug resistance (MDR), the major cause of chemotherapy failure, is a process whereby cancer cells treated with one drug develop resistance to a wide variety of unrelated agents to which the cells have not been exposed. It was recently reported that the human major vault protein (MVP) is overexpressed in a class of MDR cell lines whose drug resistance cannot be explained by the expression of known drug efflux pumps (p-glycoprotein and MDR-associated protein). Furthermore, MVP showed the greatest individual value as a marker for in vitro resistance of human cancer cell lines to a broad variety of drugs and MVP expression appears to have a high predictive value for resistance to chemotherapy in acute myeloid leukemia and ovarian carcinoma. Molecular, cell biological and immunological tools will be applied to test the hypothesis that vault expression is responsible for MDR. Vault induction in cancer cells will be examined and it will be determined whether vaults act directly to protect these cells from drug toxicity. Protein expression and antisense technology will be used to interfere with vault structure and function in MDR cells. If these manipulations lead to increased drug sensitivity they will establish a causative relationship between vault expression and MDR.